Differential amplifier circuit for a capacitive acoustic transducer and corresponding capacitive acoustic transducer

ABSTRACT

An amplifier circuit, for a capacitive acoustic transducer defining a sensing capacitor that generates a sensing signal as a function of an acoustic signal, has a first input terminal and a second input terminal, which are coupled to the sensing capacitor and: a dummy capacitor, which has a capacitance corresponding to a capacitance at rest of the sensing capacitor and a first terminal connected to the first input terminal; a first buffer amplifier, which is coupled at input to the second input terminal and defines a first differential output of the circuit; a second buffer amplifier, which is coupled at input to a second terminal of the dummy capacitor and defines a second differential output of the circuit; and a feedback stage, which is coupled between the differential outputs and the first input terminal, for feeding back onto the first input terminal a feedback signal, which has an amplitude that is a function of the sensing signal and is in phase opposition with respect thereto.

BACKGROUND

Technical Field

The present disclosure relates to a differential amplifier circuit for acapacitive acoustic transducer.

Description of the Related Art

As is known, an acoustic transducer of a capacitive type generallycomprises a sensing structure, designed to transduce acoustic pressurewaves into an electrical quantity (in particular, a capacitivevariation), and an electronic reading interface, designed to carry outappropriate processing operations (amongst which amplificationoperations) on said electrical quantity for supplying an electricaloutput signal (for example, a voltage).

The sensing structure in general comprises a mobile electrode, in theform of a diaphragm or membrane, arranged facing a fixed electrode, at ashort distance (the so-called “air gap”), to form the plates of asensing capacitor with a capacitance varying as a function of theacoustic pressure waves to be detected. The mobile electrode isgenerally free to move, or undergo deformation, in response to thepressure exerted by the incident acoustic waves, in this way causing thecapacitance variation of the sensing capacitor.

For example, MEMS (Micro-Electro-Mechanical System) capacitive acoustictransducers are known, in which the sensing structure is of amicromechanical type built using integrated micromachining techniquestypical of the semiconductor industry.

By way of example, FIG. 1 shows a micromechanical structure 1 of a MEMSacoustic transducer, of a known type, which comprises a structural layeror substrate 2 of semiconductor material, for example silicon, in whicha cavity 3 is made, for example via chemical etching from the back. Amembrane, or diaphragm, 4 is coupled to the structural layer 2 andcloses the cavity 3 at the top. The membrane 4 is flexible and, in use,undergoes deformation as a function of the pressure of incident acousticwaves.

A rigid plate 5 (generally known as “back-plate”) is arranged facing themembrane 4, in this case above it, via interposition of spacers 6 (forexample of insulating material, such as silicon oxide). The rigid plate5 constitutes the fixed electrode of a sensing capacitor with variablecapacitance, the mobile electrode of which is constituted by themembrane 4, and has a plurality of holes 7, designed to enable freecirculation of air towards the membrane 4 (rendering the rigid plate 5in effect acoustically transparent).

The micromechanical structure 1 further comprises (in a way notillustrated) electrical membrane and rigid-plate contacts, used forbiasing the membrane 4 and the rigid plate 5 and acquiring a sensingsignal indicative of the capacitive variation due to deformation of themembrane 4 caused by incident acoustic pressure waves. Typically, theseelectrical contacts are arranged in a surface portion of the die inwhich the micromechanical structure is provided.

In general, the sensing structure of capacitive acoustic transducers ischarge-biased, usually via a fixed charge. In particular, a DC biasingvoltage is applied, usually from a charge-pump stage (the higher thisvoltage, the greater the sensitivity of the microphone), and ahigh-impedance element is inserted (with impedance of the order ofteraohms, for example between 100 GΩ and 10 TΩ) between the charge-pumpstage and the sensing structure.

This high-impedance element may, for example, be provided by a pair ofdiodes arranged in back-to-back configuration, i.e., connected togetherin parallel, with the cathode terminal of one of the two diodesconnected to the anode terminal of the other, and vice versa, or by aseries of pairs of diodes in the back-to-back configuration. Thepresence of this high impedance “insulates” the DC charge stored in thesensing structure from the charge-pump stage, for frequencies higherthan a few hertz.

Since the amount of charge is fixed, an acoustic signal (acousticpressure), which impinges upon the mobile electrode of the sensingstructure, modulates the gap with respect to the rigid electrode,producing a capacitive variation and consequently a voltage variation.

This voltage is processed, in the electronic interface, by an electronicamplifier circuit, which is required to have a high input impedance (toprevent perturbation of the charge stored in the micromechanicalstructure), and then is converted into a low-impedance signal (designedto drive an external load).

FIG. 2a shows a possible embodiment of the amplifier circuit, designatedby 10, in this case with a single output, of a so-called “single-ended”type.

The sensing structure of the capacitive acoustic transducer, designatedas a whole by 11, is represented schematically by a sensing capacitor 12a with capacitance C_(MIC), which varies as a function of the acousticsignal detected, connected in series to a voltage generator 12 b, whichsupplies a sensing voltage V_(SIG) (having, in the example illustratedin FIG. 2, a generically sinusoidal waveform).

Given that, typically, the mobile electrode has a high parasiticcapacitance towards the substrate (comparable with the capacitance ofthe sensing capacitor of the sensing structure), whereas the rigidelectrode has a lower parasitic capacitance, the mobile electrode isgenerally electrically connected to a first low-impedance input terminalN1, for example to a reference ground voltage of the circuit, whereasthe rigid electrode is electrically connected to a second input terminalN2, on which the sensing voltage V_(SIG), indicative of the capacitivevariations of the sensing capacitor 12 a, is acquired.

The second input terminal N2 is further electrically connected to abiasing stage, for example a charge-pump biasing stage (here notillustrated), by interposition of a first high-impedance insulatingelement 13, constituted by a pair of diodes arranged in back-to-backconfiguration, for receiving a biasing voltage V_(CP).

The amplifier circuit 10 further comprises a decoupling capacitor 14,and an amplifier 15, in buffer or voltage-follower single-endedconfiguration (i.e., with its inverting input connected to the singleoutput). For example, the amplifier 15 is a class-A, or else a class-AB,operational amplifier.

The decoupling capacitor 14 (which operates to decouple the DC componentand couple the detected signal) is connected between the second inputterminal N2 and the non-inverting input of the amplifier 15, whichfurther receives an operating voltage V_(CM) from an appropriatereference-generator stage (here not illustrated), via interposition of asecond high-impedance insulating element 16, constituted by a respectivepair of diodes arranged in back-to-back configuration.

The operating voltage V_(CM) is a DC biasing voltage, appropriatelyselected for setting the operating point of the amplifier 15. Thisoperating voltage V_(CM) is chosen, for example, in a range comprisedbetween a supply voltage of the amplifier 15 (not shown) and thereference ground voltage.

During operation of the capacitive acoustic transducer, the (AC) sensingvoltage V_(SIG) is thus superimposed on this DC operating voltageV_(CM).

The amplifier 15 supplies on the single output OUT an output voltageV_(OUT), as a function of the sensing voltage V_(SIG) detected by thesensing structure 11 of the capacitive acoustic transducer. The outputvoltage V_(OUT) has, in the example, a sinusoidal waveform thatcorresponds in amplitude to the sensing voltage V_(SIG) (as representedschematically in FIG. 2a ).

FIG. 2b shows a further embodiment of a known-type amplifier circuit 10,which also in this case has a single-ended output.

The amplifier circuit 10 here comprises a MOS transistor 17 (in theexample of a PMOS type) in source-follower configuration, which has itsgate terminal connected to the second input terminal N2 via thedecoupling capacitor 14, its source terminal that supplies the outputvoltage V_(OUT) on the single output OUT, and its drain terminalconnected to the reference ground voltage.

The source terminal of the MOS transistor 17 further receives a biasingcurrent I_(B) from a current generator 18 connected to a line set at asupply voltage V_(cc). The second insulating element 16 in this casecouples the gate terminal of the MOS transistor 17 to the groundreference.

In general, the single-ended circuit configuration presents somedrawbacks, amongst which a poor rejection in regard to any common-modecomponent of disturbance, for example, deriving from power-supply noiseor cross-talk from nearby devices with time-varying signals.

To overcome these drawbacks, it has been proposed to replace thesingle-ended solution with a configuration, defined as “pseudo-balanced”or “pseudo-differential”, which is illustrated in FIGS. 3a and 3 b.

This solution envisages that the amplifier circuit 10 comprises a dummycapacitor 19, constituted by a capacitor of a classic type, for examplemetal-oxide-metal (MOM) or metal-insulator-metal (MIM), having acapacitance C_(DUM), with a nominal value substantially equal to thecapacitance at rest (i.e., in the absence of external stresses) C_(MIC)of the sensing capacitor 12 a.

The amplifier circuit 10 has in this case the exact duplication of thecircuit elements previously described with reference to FIGS. 2a and 2b(the duplicated elements are distinguished with a prime sign in FIGS. 3aand 3b and are not described again), for generation on a further outputOUT′ of a DC output voltage V_(out) _(_) _(DUM) designed to balance theoutput voltage V_(out), thus enabling elimination of the common-modedisturbance. Basically, two altogether equivalent circuit paths arecreated.

Also this solution is, however, not free from drawbacks.

In particular, given that the contribution of the sensing signal ispresent only on one of the two circuit paths, i.e., the one that goesfrom the sensing capacitor 12 a to the output OUT (thereby, the “pseudo”differential nature of the amplifier circuit 10), on the same output OUTa greater voltage swing is required, in particular with a value twicethat of a fully differential solution (where half of the swing would bepresent on the output OUT and the other half of the swing, in phaseopposition, on the further output OUT′).

A higher value of the supply voltage V_(cc) is thus required, withconsequent increase in power consumption.

To overcome the above problem related to the swing on the output of theamplifier, a further solution that has been proposed, illustrated inFIG. 4, envisages use of a differential amplifier 25 with four inputsand two outputs, the so-called DDA (Differential Difference Amplifier),having a differential and unity-gain architecture (the voltagedifference between the differential output terminals, which are heredesignated by Out1 and Out2, is equivalent to the voltage differencebetween the differential input terminals, which are here designated by25 a and 25 c).

The structure of the differential amplifier 25 is described in detailfor example in:

“A versatile building block: the CMOS differential differenceamplifier”, E. Sackinger, W. Guggenbuhl, IEEE Journal of Solid-StateCircuits, Vol. 22, April 1987; or

“A CMOS Fully Balanced Differential Difference Amplifier and ItsApplications”, H. Alzaher, M. Ismail, IEEE TCAS-II: Analog and DigitalSignal Processing, Vol. 48, No. 6, June 2001.

In particular, the second input terminal N2 is in this case connected,via interposition of the decoupling capacitor 14, to a firstnon-inverting input 25 a of the differential amplifier 25, a firstinverting input 25 b of which is directly feedback-connected to a firstdifferential output terminal Out₁.

Likewise, the dummy capacitor 19 has a respective first terminal,designated by N1′, connected to the ground reference terminal, and asecond terminal N2′ connected, via interposition of a respectivedecoupling capacitor 14′, to a second inverting input 25 c of thedifferential amplifier 25, a second non-inverting input 25 d of which isfurther directly feedback-connected to a second differential outputterminal Out₂ (the output voltage V_(out) being present between thefirst and second differential output terminals Out₁, Out₂).

The respective second input terminal N2′ of the dummy capacitor 19further receives the biasing voltage V_(CP) through a respective firstinsulating element 13′, constituted by a pair of diodes arranged inback-to-back configuration and receiving the biasing voltage V_(CP).Likewise, the second inverting input 25 c receives the operating voltageV_(CM), via a respective second high-impedance insulating element 16′,which in the example is also constituted by a pair of diodes arranged inback-to-back configuration (the operating voltage V_(CM) is thus acommon-mode voltage for the first non-inverting input 25 a and for thesecond inverting input 25 c of the differential amplifier 25).

The dummy capacitor 19, in this case, enables creation of asubstantially balanced path for the buffer inputs (i.e., thenon-inverting input 25 a and the inverting input 25 c) of thedifferential amplifier 25, for a better common-mode noise rejection.

Even though it enables improvement of the capacity of disturbancerejection, also the differential configuration described with referenceto FIG. 4 has some drawbacks.

In particular, the above solution involves two differential inputstages, with a consequent increase in noise and current consumption. Ithas a wide common-mode input interval on account of the differentsignals present on the four inputs of the differential amplifier 25. Thetransistors of each input stage are driven by a differential signalhaving a large amplitude, i.e., the virtual-ground principle does notapply (as is known to a person skilled in the sector), with a consequenthigh distortion for signals of large amplitude. Finally, thedifferential signal is effectively present only on the differentialoutput terminals Out₁, Out₂, whereas the input terminals are not fullydifferential.

In general, the need is thus felt to provide an amplifier circuit for acapacitive acoustic transducer that will enable the disadvantages andlimitations associated to known solutions to be overcome, at least inpart.

BRIEF SUMMARY

One aim of the present disclosure is to meet the aforesaid need, and inparticular to provide an amplifier circuit of a fully differential typefor a capacitive acoustic transducer.

Consequently, according to the present disclosure an amplifier circuitfor a capacitive acoustic transducer and a corresponding capacitiveacoustic transducer are provided, as defined in the annexed claims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

For a better understanding of the present disclosure, a preferredembodiment thereof is now described purely by way of non-limitingexample and with reference to the attached drawings, wherein:

FIG. 1 is a schematic cross-sectional view of a micromechanicalstructure of a known capacitive acoustic transducer, of a MEMS type;

FIGS. 2a and 2b show circuit diagrams of a single-output amplifiercircuit, of a known type, for the capacitive acoustic transducer;

FIGS. 3a and 3b show circuit diagrams of pseudo-differential amplifiercircuits of a known type for the capacitive acoustic transducer;

FIG. 4 is a circuit diagram of a differential amplifier circuit of aknown type for the capacitive acoustic transducer;

FIG. 5 is a circuit diagram of a fully differential amplifier circuitfor the capacitive acoustic transducer, according to one embodiment ofthe present solution; and

FIG. 6 is a general block diagram of an electronic device incorporatingthe capacitive acoustic transducer, according to a further aspect of thepresent solution.

DETAILED DESCRIPTION

As illustrated in FIG. 5, one aspect of the present solution envisagesproviding an amplifier circuit 30 for a capacitive acoustic transducer(obtained as described previously) of a fully differential (or fullybalanced) type.

The amplifier circuit 30 once again comprises: the high-resistanceinsulating element 13, for example constituted by a pair of diodesarranged in back-to-back configuration, connected between a biasing line31, which receives the biasing voltage V_(CP), for example from acharge-pump stage (here not illustrated) and, in this case, the firstinput terminal N1 designed to be coupled to the sensing capacitor 12 a;and the dummy capacitor 19, which in this case has a first terminal N1′connected to the same first input terminal N1, and a second terminalN2′.

The amplifier circuit 30 further comprises a first amplifier 34 and asecond amplifier 35, in buffer or voltage-follower single-endedconfiguration (i.e., with single output and with inverting inputconnected to the single output, these amplifiers being referred to inwhat follows as “buffer amplifiers”). For example, the first and secondbuffer amplifiers 34, 35 have unity gain and are in source-followerconfiguration.

The output terminals of the buffer amplifiers 34, 35 define the firstand second output terminals Out₁, Out₂ of the amplifier circuit 30,between which the output voltage V_(out) is present, the value of whichis a function of the sensing voltage V_(SIG) generated by the sensingstructure 11 of the capacitive acoustic transducer in response toacoustic stresses.

In greater detail, the second input terminal N2, which is designed to beconnected to the sensing capacitor 12 a and on which the sensing voltageV_(SIG) is present, is connected to the non-inverting input of the firstbuffer amplifier 34, and the second terminal N2′ of the dummy capacitor19 is connected to the non-inverting input of the second bufferamplifier 35.

Furthermore, the non-inverting inputs of the first and second bufferamplifiers 34, 35 are connected to a respective biasing line 32, fromwhich they receive the operating voltage V_(CM) supplied by anappropriate reference-generator stage (here not illustrated) viainterposition of a respective high-impedance insulating element 16, 16′constituted by a respective pair of diodes arranged in back-to-backconfiguration. As discussed previously, the operating voltage V_(CM) isan appropriate DC biasing voltage, which sets the working point of thebuffer amplifiers 34 and 35 (i.e., the reference voltage at input to thesame buffer amplifiers 34, 35).

The amplifier circuit 30 further comprises: a resistive divider 38,formed by a first division resistor 38 a and by a second divisionresistor 38 b, which are connected in series between the first andsecond output terminals Out₁, Out₂ and define between them a feedbacknode N_(R); a feedback amplifier 40, for example a high-gain OTA(Operational Transconductance Amplifier), which has its inverting inputconnected to the feedback node N_(R), its non-inverting input which alsoreceives the operating voltage V_(CM), and a feedback resistor 42connected between its inverting input and a corresponding output; and afeedback capacitor 44, which is connected between the output of thefeedback amplifier 40 and the first input terminal N1, and has acapacitance C_(B) much higher than the sum of the capacitances C_(MIC)and C_(DUM) of the sensing capacitor 12 a and of the dummy capacitor 19:C _(B) >>C _(MIC) +C _(DUM).

For example, capacitances C_(MIC) and C_(DUM) are in the region of 1 pF,whereas the capacitance C_(B) is in the region of 10-20 pF.

In particular, for the reasons that will be discussed hereinafter, thevalue of resistance R₁ of the first division resistor 38 a satisfies thefollowing relation with the value of resistance R₂ of the seconddivision resistor 38 b and with the value of resistance R_(B) of thefeedback resistor 42:R ₁ =R ₂ //R _(B).

The amplifier circuit 30 has a fully differential configuration, both atinput and at output in so far as it has two outputs, at the outputterminals Out₁, Out₂, which are in phase opposition, in particular witha phase shift of 180°, with respect to one another, the difference ofwhich define the output voltage V_(out); and two inputs, at thenon-inverting inputs of the first and second buffer amplifiers 34, 35,which are also in phase opposition, in particular with a phase shift of180°, with respect to one another.

During operation, the virtual short-circuit at input to the feedbackamplifier 40 and the feedback action cause the voltage on the feedbacknode N_(R) to be equal to the operating voltage V_(CM) (superimposed onwhich is an oscillation of negligible value, since the gain of thefeedback amplifier 40 is assumed as being very high).

Therefore, the first and second output terminals Out₁, Out₂ are atvoltages of equal amplitude and in phase opposition (given that thevoltage on the feedback node N_(R) is given by the half-sum of the samevoltages). In other words, the voltage on the feedback node N_(R) is thecommon-mode voltage between the output terminals Out₁, Out₂.

In particular, the voltage on the first output terminal Out₁ is equal to+V_(SIG)/2, whereas the voltage on the second output terminal Out₂ isequal to −V_(SIG)/2.

Furthermore, it may be easily shown that the bottom plate of thefeedback capacitor 44, connected to the output of the feedback amplifier40, is set at a feedback voltage V_(R) with a value of −V_(SIG)/2, andthe sinusoidal variation of this voltage is fed back, substantiallyunchanged, onto the first input terminal N1, given the relation betweenthe values of capacitance of the feedback capacitor 44, of the sensingcapacitor 12 a, and of the dummy capacitor 19.

In other words, the voltage on the bottom plate of the feedbackcapacitor 44 varies, phase-shifted by 180°, of a value equal to half ofthe sensing voltage V_(SIG), consequently shifting the voltages on thenon-inverting input of each of the first and second buffer amplifiers34, 35.

Consequently, on the non-inverting node of the first buffer amplifier 34a voltage equal to +V_(SIG)/2 is present (given by the differencebetween the sensing voltage V_(SIG) and the voltage fed back onto thefirst input terminal N1), whereas on the non-inverting node of thesecond buffer amplifier 35 a voltage equal to −V_(SIG)/2 is present.

In other words, a purely, or truly, differential signal is present bothbetween the inputs and between the outputs of the amplifier circuit 30.

The expression referred to previously for the relation between the valueof resistance R₁ of the first division resistor 38 a, the value ofresistance R₂ of the second division resistor 38 b, and the value ofresistance R_(B) of the feedback resistor 42 is now discussed.

For this purpose, the current coming out of the feedback node N_(R) isdesignated by I_(S1), and the current that circulates in the feedbackresistor 42 is designated by I_(S2). The following expressions apply:I _(S1) =V _(SIG)/2R ₁ −V _(SIG)/2R ₂I _(S2) =V _(SIG)/2R _(B)

However, I _(S2) =I _(S1), then:V _(SIG)/2R ₁ −V _(SIG)/2R ₁ =V _(SIG)/2R _(B)

and consequently1/R ₁−1/R ₂=1/R _(B)1/R ₁=1/R ₂+1/R _(B)

i.e.,R ₁ =R ₂ //R _(B).

The advantages of the solution proposed are clear from the foregoingdescription.

In any case, it is emphasized once again that the amplifier circuit 30for the capacitive acoustic transducer provides a fully differentialsolution, with input and output signals in phase opposition.

The above solution thus enables the disadvantages of known solutions tobe overcome thanks to the following. The principle of a virtualshort-circuit at the inputs of the amplifiers, in particular the bufferamplifiers 34, 35, is respected. Differential signals are present alsoat the inputs of the amplifier circuit 30, not only at the outputs. Useof a complex circuit structure (for example, of the type described withreference to FIG. 4, having two differential input stages) is notrequired, thus preventing the associated harmonic distortions, thetrade-off required with noise, and signal attenuation.

The solution proposed does not require any modification to themanufacturing method or to the technology used for manufacturing theacoustic transducer, for example of a MEMS type, as compared totraditional solutions.

The above features thus make the use of the acoustic transduceradvantageous in an electronic device 50, in particular of a portabletype, as illustrated schematically in FIG. 6.

In FIG. 6, designated by 51 is the capacitive acoustic transducer, forexample of a MEMS type, which may include, within a same package 52, thesensing structure 11, comprising, for example, an appropriatemicromechanical structure, and the reading interface circuit includingthe amplifier circuit 30, supplying the output voltage V_(out); thereading interface circuit may be obtained in the same die as that inwhich the micromechanical structure is provided, or in a different die,which may in any case be housed in the same package 52.

The electronic device 50 is, for example, a portable mobilecommunication device, such as a cellphone, a PDA (personal digitalassistant), a portable computer, but also a digital audio player withvoice-recording capacity, a photographic or video camera, a controllerfor videogames, etc. The electronic device 50 is generally able toprocess, store, and/or transmit and receive signals and information.

The electronic device 50 further comprises a microprocessor 54, whichreceives the signals detected by the acoustic transducer 51 (the outputvoltage V_(out), possibly being further processed), and an input/output(I/O) interface 55, for example provided with a keypad and a display,connected to the microprocessor 54.

Furthermore, the electronic device 50 may comprise a speaker 57, forgenerating sounds on an audio output (not shown), and a non-volatileinternal memory 58.

Finally, it is clear that modifications and variations may be made towhat has been described and illustrated herein, without therebydeparting from the scope of the present disclosure.

In particular, the solution described may find advantageous applicationboth for analog acoustic transducers and for digital acoustictransducers.

Furthermore, as previously highlighted, the solution described may applyalso to different types of acoustic transducers, for example to ECMs(Electret Condenser Microphones), comprising, in a known way, adeformable conductive membrane capacitively coupled to a fixed electrodeor plate.

The various embodiments described above can be combined to providefurther embodiments. All of the U.S. patents, U.S. patent applicationpublications, U.S. patent applications, foreign patents, foreign patentapplications and non-patent publications referred to in thisspecification and/or listed in the Application Data Sheet areincorporated herein by reference, in their entirety. Aspects of theembodiments can be modified, if necessary to employ concepts of thevarious patents, applications and publications to provide yet furtherembodiments.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

The invention claimed is:
 1. An amplifier circuit, comprising: a firstinput terminal and a second input terminal designed to be electricallycoupled to a first terminal and a second terminal of a sensing capacitorconfigured to generate a sensing signal as a function of an acousticsignal and being part of a sensing structure of a capacitive acoustictransducer; a dummy capacitor having a capacitance corresponding to arespective capacitance at rest in the absence of said acoustic signal,of the sensing capacitor and a respective first terminal connected tothe first input terminal; a first buffer amplifier having itsnon-inverting input terminal coupled to the second input terminal andits inverting input terminal connected to a respective output terminal,which defines a first differential output of said amplifier circuit; asecond buffer amplifier having its non-inverting input terminal coupledto a second terminal of the dummy capacitor and its inverting inputterminal connected to a respective output terminal, which defines asecond differential output of said amplifier circuit, an output signalas a function of said sensing signal being present in use between saidfirst and second differential outputs; and a feedback stage coupledbetween said first and second differential outputs and said first inputterminal and configured to feed back onto said first input terminal afeedback signal, which has an amplitude that is a function of saidsensing signal and is in phase opposition with respect to said sensingsignal.
 2. The amplifier circuit according to claim 1, wherein saidfirst and second buffer amplifiers are unity-gain voltage followers. 3.The amplifier circuit according to claim 1, wherein said feedback signalhas an amplitude equal to half the amplitude of said sensing signal andis phase-shifted by 180° with respect to said sensing signal.
 4. Theamplifier circuit according to claim 1, wherein said feedback stagecomprises: a resistive divider which is connected between said firstdifferential output and said second differential output and defines afeedback node; a feedback amplifier which has a first input terminalconnected to the feedback node, a second input terminal receiving areference voltage, and an output terminal; and a feedback capacitorconnected between the output terminal of said feedback amplifier andsaid first input terminal.
 5. The amplifier circuit according to claim4, wherein said reference voltage is a common-mode voltage between saidfirst and second differential outputs.
 6. The amplifier circuitaccording to claim 4, comprising a feedback resistor connected betweenthe first input terminal and the output terminal of said feedbackamplifier.
 7. The amplifier circuit according to claim 6, wherein saidfeedback resistor has a resistance R_(B); and wherein said resistivedivider comprises a first division resistor, which is connected betweensaid first differential output and said feedback node and has aresistance R₁, and a second division resistor, which is connectedbetween said second differential output and said feedback node and has aresistance R₂; wherein:R ₁ =R ₂ //R _(B).
 8. The amplifier circuit according to claim 4,wherein said feedback capacitor has a capacitance C_(B), said sensingcapacitor has a capacitance C_(MIC), and said dummy capacitor has acapacitance C_(DUM); wherein:C _(B) >>C _(MIC) +C _(DUM).
 9. The amplifier circuit according to claim4, comprising a first high-impedance element that couples said firstinput terminal to a first biasing line set at a biasing voltage; and asecond high-impedance element and a third high-impedance element, whichcouple, respectively, said second input terminal and said secondterminal of said dummy capacitor, to a second biasing line set at saidreference voltage.
 10. A capacitive acoustic transducer, comprising: asensing structure which defines a sensing capacitor and is configured togenerate a sensing signal as a function of an acoustic signal; and anamplifier circuit configured to process said sensing signal and supplyan output signal, the amplifier circuit including, a first input nodeand a second input node coupled to first and second nodes of the sensingcapacitor, respectively; a dummy capacitor having a first node coupledto the first input node, the dummy capacitor having a capacitance valuethat is approximately equal to a capacitance value of the sensingcapacitor in the absence of the acoustic signal; a first bufferamplifier circuit having a non-inverting input coupled to the secondinput node and an output that defines a first differential output of theamplifier circuit; a second buffer amplifier circuit having anon-inverting input coupled to a second node of the dummy capacitor andan output that defines a second differential output of the amplifiercircuit; and a feedback circuit coupled between the first and seconddifferential outputs and the first input node, the feedback circuitconfigured to generate on the first input node a feedback signal havingan amplitude that is a function of the sensing signal and having a phasein phase opposition with the sensing signal.
 11. The capacitive acoustictransducer of claim 10, wherein the capacitive acoustic transducercomprises a MEMS type transducer.
 12. The capacitive acoustic transducerof claim 10, wherein the feedback circuit comprises a resistive voltagedivider coupled across the first and second differential outputs.
 13. Anelectronic device, comprising: a capacitive acoustic transducer providedwith a sensing structure which defines a sensing capacitor and isconfigured to generate a sensing signal as a function of an acousticsignal; an amplifier circuit including, a first input terminal and asecond input terminal designed to be electrically coupled to a firstterminal and a second terminal of the sensing capacitor; a dummycapacitor having a capacitance corresponding to a respective capacitanceat rest, in the absence of the acoustic signal, of the sensingcapacitor, and a respective first terminal connected to the first inputterminal; a first buffer amplifier having a non-inverting input terminalcoupled to the second input terminal and an inverting input terminalconnected to a respective output terminal that defines a firstdifferential output of the amplifier circuit; a second buffer amplifierhaving a non-inverting input terminal coupled to a second terminal ofthe dummy capacitor and an inverting input terminal connected to arespective output terminal that defines a second differential output ofthe amplifier circuit, wherein an output signal, as a function of thesensing signal, being present in use between the first and seconddifferential outputs; and a feedback stage coupled between the first andsecond differential outputs and the first input terminal, and configuredto feed back onto the first input terminal a feedback signal having anamplitude that is a function of the sensing signal and is in phaseopposition with respect to the sensing signal; and a microprocessor unitcoupled to the amplifier circuit of the capacitive acoustic transducer.14. The electronic device according to claim 13, wherein the electronicdevice comprises one of a cellphone; a personal digital assistant; aportable computer; a digital audio player with voice-recording capacity;a photographic camera or video camera; and a control device forvideogames.
 15. The electronic device according to claim 14 furthercomprising an input/output interface and a memory coupled to themicroprocessor unit.
 16. The electronic device according to claim 14,wherein each of the first and second buffer amplifiers comprises anoperational amplifier configured in a voltage-follower configuration.17. The electronic device according to claim 16, wherein the feedbackstage comprises a resistive voltage divider coupled between the firstand second differential outputs and a feedback amplifier circuit havingan input coupled between a node of the voltage divider and an outputcoupled through feedback capacitor to the first input terminal.
 18. Theelectronic device according to claim 17, wherein the feedback amplifiercomprises an operational transconductance amplifier.
 19. A processingmethod, comprising: providing a capacitive acoustic transducer with asensing structure which defines a sensing capacitor and is configured togenerate a sensing signal as a function of an acoustic signal; providingan amplifier circuit having a first input terminal and a second inputterminal which are designed to be electrically coupled to a firstterminal and a second terminal of said sensing capacitor the amplifiercircuit comprising: a dummy capacitor which has a capacitancecorresponding to a respective capacitance at rest in the absence of saidacoustic signal, of the sensing capacitor and a respective firstterminal connected to the first input terminal; a first buffer amplifierhaving its non-inverting input terminal coupled to the second inputterminal and its inverting input terminal connected to a respectiveoutput terminal, which defines a first differential output of saidamplifier circuit; and a second buffer amplifier having itsnon-inverting input terminal coupled to a second terminal of the dummycapacitor and its inverting input terminal connected to a respectiveoutput terminal, which defines a second differential output of saidamplifier circuit, an output signal which is a function of said sensingsignal being present between said first and second differential outputsand feeding back onto said first input terminal a feedback signal whichhas an amplitude that is a function of said sensing signal and is inphase opposition with respect to the sensing signal thereby implementinga fully differential processing of said sensing signal.
 20. The methodaccording to claim 19 further comprising: generating a feedback voltagethat is equal to a common-mode voltage between the first and seconddifferential outputs; and generating the feedback signal based on thefeedback voltage, wherein said feedback signal has an amplitude equal tohalf of the amplitude of said sensing signal and is phase-shifted by180° with respect to said sensing signal.